Heretofore, switching power supply devices are known. For example, PTL 1 discloses a semiconductor device that includes: a metal substrate; an insulator formed so as to cover one surface of the metal substrate; a positive conductor formed on the insulator; a negative conductor formed on the insulator; a group of switching elements that is formed on the insulator and that is connected to the positive conductor and to the negative conductor; and a loop conductor that is electrically insulated, by the insulator, from the positive conductor and the negative conductor. Further, the loop conductor is disposed in substantially parallel to the positive conductor and to the negative conductor. In this semiconductor device, when a direct electric current flows through the positive conductor and the negative conductor, an induced electric current is induced in the loop conductor. Then, the induced electric current induced in the loop conductor generates a magnetic field, which weakens a magnetic field generated by the direct electric current flowing through the positive conductor and the negative conductor. In this way, an inductance (total inductance) of the positive and negative conductors is reduced.